1. Field of the Invention
The present invention relates to an optical module applicable to a backlight unit of a liquid crystal display.
2. Discussion of the Related Art
As information technology develops, the demand for display devices that connect media between users and information is increasing. Hence, the need for flat panel displays (FPDs) such as liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), and plasma display panels (PDPs), is increasing. Among them, liquid crystal displays are widely used which can implement a high resolution and become bigger as well as smaller.
Liquid crystal displays display an image by using the electrooptical characteristics of liquid crystal.
For this, liquid crystal display may include a liquid crystal panel and a backlight unit. The liquid crystal panel may display an image using a light provided from the backlight unit.
The backlight unit may be disposed under the liquid crystal panel to provide light to the liquid crystal panel. Such backlight units may be classified into edge type and direct type according to the position of a light source.
The edge type backlight unit refers to a backlight unit having a light source disposed in proximity to a side face of a liquid crystal panel. In the edge type backlight unit, a light emitted from the light source may be guided by a light guide plate arranged on the back surface of the liquid crystal panel, and then provided to the liquid crystal panel.
The edge type backlight unit is commonly used for small to medium size liquid crystal displays because it is relatively advantageous in thinning.
The direct type backlight unit refers to a backlight unit having a plurality of light sources disposed on the back surface of a liquid crystal panel. In the direct type backlight unit, as described above, the light sources are disposed on the back surface of the liquid crystal panel, and thus a light emitted from the light source directly irradiates to the liquid crystal panel without any help from a light guide plate. The direct type backlight unit includes a diffusion plate disposed on the back surface of the liquid crystal panel in order to prevent the shape of the light source from being viewed through the liquid crystal panel.
The direct type backlight unit is commonly used for large-size liquid crystal displays requiring a high intensity because it can use relatively many light sources compared to the edge type backlight unit.
Conventionally, cold cathode fluorescent lamps (CCFLs) have stable illumination and brightness characteristics and emitting white light. These CCFLs have been commonly used as the light source of edge type and direct type backlight units.
However, the CCFLS make difficult to have a limitation in thinning, require a high power consumption, and have a low color reproducibility.
Thereupon, in recent years, point light sources have been used as the light source that each of edge type and direct type backlight units is provided with. Here, the point light sources may include, for example, red/green/blue light emitting diodes (LEDs). Monochromatic lights emitted from the red/green/blue light emitting diodes, respectively, match well with color filters of the liquid crystal panel, and this can enhance color reproducibility.
However, a light emitted from the point light sources have strong straightforwardness, and thus the point light sources are not used alone but along with an optical lens for refracting a light emitted from the point light sources and emitting it. In other words, the point light sources and the optical lens may be configured in one package. The optical lens will be described more concretely with reference to FIG. 1.
FIG. 1 is a perspective view schematically showing a related art optical lens.
Referring to FIG. 1, the related art optical lens 30 can symmetrically refract and emit a light incident from point light sources to be disposed thereunder. Here, the optical lens 30 symmetrically refracts and emits the light incident from the point light sources in order to ensure uniform luminance distribution depending on the angle of direction by emitting the light in every angle of direction.
For this, the optical lens 30 may include an exterior curved surface 40 that is exposed to the outside and formed in a reversed cone shape at the top part. This exterior curved surface 40 may be formed symmetrically according to the direction of viewing the exterior curved surface 40. In other words, the shape of the exterior curved shape 40 viewed on a cross section taken at a right angle through the optical lens 30 may be all the same regardless of a direction for cutting the optical lens 30 at a right angle.
FIG. 2 is a view showing a result of simulating the degree of color mixing using the optical lens as shown in FIG. 1.
If the exterior curved surface 40 of the optical lens 30 is symmetrically formed, as shown in FIG. 2, the degree of color mixing of monochromatic colors emitted from point light sources including red/green/blue light emitting diodes, may be reduced. Due to this, in the edge type backlight unit having an optical lens 30 with an exterior curved shape 40 symmetrically formed, there is a problem of non-uniform luminance at a light incident portion of a light guide plate positioned in proximity to the optical lens 30. Further, the edge type backlight unit may have a problem of low light efficiency. This is because, in the edge type backlight unit, the optical lens 30 is disposed in proximity to the light incident portion of the light guide plate, thus not requiring any light emitted from substantially every angle of direction.
Meanwhile, in the direct type backlight unit having an optical lens 30 with an exterior curved shape 40 symmetrically formed, there is a problem of having to additionally arrange a diffusion member between the optical lens 30 and a diffusion plate in order to enhance the degree of color mixing. Due to this, there may be a limitation in achieving the thinning of the direct type backlight unit. Further, in the direct type backlight unit, a light emitted from the point light sources may be laterally inclined. Accordingly, the direct type backlight unit has the problem of having to increase the haze process of the diffusion plate in order to direct the light at right angles. This may lead to the problem of low light efficiency in the direct type backlight unit.